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 Austin
 Drainage Criteria Manual
 Section 2. DETERMINATION OF STORM RUNOFF
 Appendix 2.5.0. THE SOIL CONSERVATION SERVICE METHOD FOR CALCULATION OF PEAK FLOWS

§ 2.5.1. Austin Twenty-Four (24) Hour Storm Rainfall Distributions  

Latest version.

  • The City of Austin has adopted the use of an SCS 24-hour storm duration with a Type III distribution for use with the SCS method. The DDF and IDF values to be used for the Austin area are shown in Table 2-3 and 2-4 above. For use in spreadsheet calculations, Table 2-6 below provides the Type III distribution ordinates in 5-minute increments as derived from the HEC-HMS program. The ordinates should be multiplied by the total 24 hour precipitation depth to produce the design rainfall distribution. When using the HEC-HMS model, the computational time interval should be selected based on criteria for the minimum lag time. The maximum computational time interval used in a HEC-HMS model should be 6 minutes.

    Table 2-6
    Type III Distribution Ordinates In 5-Minute Time Increment

    Time Incremental Cumulative Time Incremental Cumulative Time Incremental Cumulative
    0:00 0.0000 0.0000 8:05 0.0023 0.1163 16:10 0.0021 0.8903
    0:05 0.0008 0.0008 8:10 0.0022 0.1185 16:15 0.0021 0.8924
    0:10 0.0009 0.0017 8:15 0.0023 0.1208 16:20 0.0020 0.8944
    0:15 0.0008 0.0025 8:20 0.0025 0.1233 16:25 0.0020 0.8964
    0:20 0.0008 0.0033 8:25 0.0025 0.1258 16:30 0.0020 0.8984
    0:25 0.0009 0.0042 8:30 0.0026 0.1284 16:35 0.0019 0.9003
    0:30 0.0008 0.0050 8:35 0.0027 0.1311 16:40 0.0019 0.9022
    0:35 0.0008 0.0058 8:40 0.0028 0.1339 16:45 0.0019 0.9041
    0:40 0.0009 0.0067 8:45 0.0028 0.1367 16:50 0.0018 0.9059
    0:45 0.0008 0.0075 8:50 0.0030 0.1397 16:55 0.0019 0.9078
    0:50 0.0008 0.0083 8:55 0.0030 0.1427 17:00 0.0017 0.9095
    0:55 0.0009 0.0092 9:00 0.0031 0.1458 17:05 0.0018 0.9113
    1:00 0.0008 0.0100 9:05 0.0032 0.1490 17:10 0.0016 0.9129
    1:05 0.0008 0.0108 9:10 0.0032 0.1522 17:15 0.0017 0.9146
    1:10 0.0009 0.0117 9:15 0.0033 0.1555 17:20 0.0016 0.9162
    1:15 0.0008 0.0125 9:20 0.0034 0.1589 17:25 0.0016 0.9178
    1:20 0.0008 0.0133 9:25 0.0035 0.1624 17:30 0.0016 0.9194
    1:25 0.0009 0.0142 9:30 0.0035 0.1659 17:35 0.0015 0.9209
    1:30 0.0008 0.0150 9:35 0.0037 0.1696 17:40 0.0015 0.9224
    1:35 0.0008 0.0158 9:40 0.0037 0.1733 17:45 0.0015 0.9239
    1:40 0.0009 0.0167 9:45 0.0038 0.1771 17:50 0.0014 0.9253
    1:45 0.0008 0.0175 9:50 0.0039 0.1810 17:55 0.0014 0.9267
    1:50 0.0008 0.0183 9:55 0.0040 0.1850 18:00 0.0013 0.9280
    1:55 0.0009 0.0192 10:00 0.0040 0.1890 18:05 0.0013 0.9293
    2:00 0.0008 0.0200 10:05 0.0041 0.1931 18:10 0.0014 0.9307
    2:05 0.0008 0.0208 10:10 0.0044 0.1975 18:15 0.0012 0.9319
    2:10 0.0009 0.0217 10:15 0.0045 0.2020 18:20 0.0013 0.9332
    2:15 0.0008 0.0225 10:20 0.0047 0.2067 18:25 0.0014 0.9346
    2:20 0.0009 0.0234 10:25 0.0048 0.2115 18:30 0.0012 0.9358
    2:25 0.0009 0.0243 10:30 0.0050 0.2165 18:35 0.0012 0.9370
    2:30 0.0009 0.0252 10:35 0.0051 0.2216 18:40 0.0013 0.9383
    2:35 0.0009 0.0261 10:40 0.0054 0.2270 18:45 0.0013 0.9396
    2:40 0.0009 0.0270 10:45 0.0055 0.2325 18:50 0.0012 0.9408
    2:45 0.0009 0.0279 10:50 0.0057 0.2382 18:55 0.0013 0.9421
    2:50 0.0010 0.0289 10:55 0.0058 0.2440 19:00 0.0012 0.9433
    2:55 0.0009 0.0298 11:00 0.0060 0.2500 19:05 0.0012 0.9445
    3:00 0.0010 0.0308 11:05 0.0064 0.2564 19:10 0.0011 0.9456
    3:05 0.0009 0.0317 11:10 0.0070 0.2634 19:15 0.0012 0.9468
    3:10 0.0010 0.0327 11:15 0.0077 0.2711 19:20 0.0012 0.9480
    3:15 0.0010 0.0337 11:20 0.0084 0.2795 19:25 0.0011 0.9491
    3:20 0.0010 0.0347 11:25 0.0089 0.2884 19:30 0.0012 0.9503
    3:25 0.0010 0.0357 11:30 0.0096 0.2980 19:35 0.0012 0.9515
    3:30 0.0010 0.0367 11:35 0.0131 0.3111 19:40 0.0011 0.9526
    3:35 0.0010 0.0377 11:40 0.0187 0.3298 19:45 0.0011 0.9537
    3:40 0.0011 0.0388 11:45 0.0261 0.3559 19:50 0.0011 0.9548
    3:45 0.0010 0.0398 11:50 0.0289 0.3848 19:55 0.0011 0.9559
    3:50 0.0010 0.0408 11:55 0.0425 0.4273 20:00 0.0011 0.9570
    3:55 0.0011 0.0419 12:00 0.0727 0.5000 20:05 0.0011 0.9581
    4:00 0.0011 0.0430 12:05 0.0727 0.5727 20:10 0.0011 0.9592
    4:05 0.0011 0.0441 12:10 0.0425 0.6152 20:15 0.0011 0.9603
    4:10 0.0011 0.0452 12:15 0.0289 0.6441 20:20 0.0010 0.9613
    4:15 0.0011 0.0463 12:20 0.0261 0.6702 20:25 0.0010 0.9623
    4:20 0.0011 0.0474 12:25 0.0187 0.6889 20:30 0.0011 0.9634
    4:25 0.0011 0.0485 12:30 0.0131 0.7020 20:35 0.0010 0.9644
    4:30 0.0012 0.0497 12:35 0.0096 0.7116 20:40 0.0010 0.9654
    4:35 0.0012 0.0509 12:40 0.0089 0.7205 20:45 0.0010 0.9664
    4:40 0.0011 0.0520 12:45 0.0084 0.7289 20:50 0.0010 0.9674
    4:45 0.0012 0.0532 12:50 0.0077 0.7366 20:55 0.0010 0.9684
    4:50 0.0012 0.0544 12:55 0.0070 0.7436 21:00 0.0010 0.9694
    4:55 0.0011 0.0555 13:00 0.0064 0.7500 21:05 0.0010 0.9704
    5:00 0.0012 0.0567 13:05 0.0060 0.7560 21:10 0.0010 0.9714
    5:05 0.0012 0.0579 13:10 0.0058 0.7618 21:15 0.0009 0.9723
    5:10 0.0013 0.0592 13:15 0.0057 0.7675 21:20 0.0010 0.9733
    5:15 0.0012 0.0604 13:20 0.0055 0.7730 21:25 0.0010 0.9743
    5:20 0.0013 0.0617 13:25 0.0054 0.7784 21:30 0.0009 0.9752
    5:25 0.0013 0.0630 13:30 0.0051 0.7835 21:35 0.0010 0.9762
    5:30 0.0012 0.0642 13:35 0.0050 0.7885 21:40 0.0009 0.9771
    5:35 0.0012 0.0654 13:40 0.0048 0.7933 21:45 0.0009 0.9780
    5:40 0.0014 0.0668 13:45 0.0047 0.7980 21:50 0.0010 0.9790
    5:45 0.0012 0.0680 13:50 0.0045 0.8025 21:55 0.0009 0.9799
    5:50 0.0013 0.0693 13:55 0.0044 0.8069 22:00 0.0009 0.9808
    5:55 0.0014 0.0707 14:00 0.0041 0.8110 22:05 0.0008 0.9816
    6:00 0.0013 0.0720 14:05 0.0040 0.8150 22:10 0.0009 0.9825
    6:05 0.0013 0.0733 14:10 0.0040 0.8190 22:15 0.0009 0.9834
    6:10 0.0014 0.0747 14:15 0.0039 0.8229 22:20 0.0009 0.9843
    6:15 0.0014 0.0761 14:20 0.0038 0.8267 22:25 0.0009 0.9852
    6:20 0.0015 0.0776 14:25 0.0037 0.8304 22:30 0.0008 0.9860
    6:25 0.0015 0.0791 14:30 0.0037 0.8341 22:35 0.0008 0.9868
    6:30 0.0015 0.0806 14:35 0.0035 0.8376 22:40 0.0009 0.9877
    6:35 0.0016 0.0822 14:40 0.0035 0.8411 22:45 0.0008 0.9885
    6:40 0.0016 0.0838 14:45 0.0034 0.8445 22:50 0.0008 0.9893
    6:45 0.0016 0.0854 14:50 0.0033 0.8478 22:55 0.0009 0.9902
    6:50 0.0017 0.0871 14:55 0.0033 0.8511 23:00 0.0007 0.9909
    6:55 0.0016 0.0887 15:00 0.0032 0.8543 23:05 0.0008 0.9917
    7:00 0.0018 0.0905 15:05 0.0030 0.8573 23:10 0.0008 0.9925
    7:05 0.0017 0.0922 15:10 0.0030 0.8603 23:15 0.0008 0.9933
    7:10 0.0019 0.0941 15:15 0.0030 0.8633 23:20 0.0008 0.9941
    7:15 0.0018 0.0959 15:20 0.0028 0.8661 23:25 0.0007 0.9948
    7:20 0.0019 0.0978 15:25 0.0028 0.8689 23:30 0.0008 0.9956
    7:25 0.0019 0.0997 15:30 0.0027 0.8716 23:35 0.0008 0.9964
    7:30 0.0019 0.1016 15:35 0.0026 0.8742 23:40 0.0007 0.9971
    7:35 0.0020 0.1036 15:40 0.0025 0.8767 23:45 0.0008 0.9979
    7:40 0.0020 0.1056 15:45 0.0025 0.8792 23:50 0.0007 0.9986
    7:45 0.0020 0.1076 15:50 0.0023 0.8815 23:55 0.0006 0.9992
    7:50 0.0021 0.1097 15:55 0.0022 0.8837 24:00 0.0008 1.0000
    7:55 0.0021 0.1118 16:00 0.0023 0.8860
    8:00 0.0022 0.1140 16:05 0.0022 0.8882